The present invention relates to a drum cleaning process and apparatus for electrostatic photography by which residual toner particles are throughly removed from a photoconductive element after a transfer operation.
In a well known dry type electrophotographic process, a photoconductive drum or belt is electrostatically charged, radiated with a light image to cause local photoconduction and produce an electrostatic image, developed by applying a toner substance thereto to form a toner image and subjected to a transfer operation in which the toner image is transferred to a copy sheet. In practical application, it is impossible to transfer the toner image to the copy sheet completely, and residual toner remains on the drum after the transfer operation.
In order to utilize the drum for sucessive copying operations of different original documents, it is necessary to remove this residual toner from the drum, since double printing would occur if it were not removed.
In a wet electrophotographic process, a scraper blade is used to remove the residual toner. However, this expedient is not applicable to a dry electrophotographic process since abrasion of the delicate photoconductive surface layer on the drum would occur due to dry friction.
In order to remove the residual toner, it is necessary to dissipate the electrostatic image on the drum which strongly binds the residual toner to the drum surface. For this reason, most conventional dry electrophotographic machines comprise a corona discharge unit to apply an electric field opposite in polarity to the initial electrostatic charge to neutralize the electrostatic image on the drum, and a light source to subsequently illuminate the drum surface to cause photoconduction and further discharge of the drum. Since the electrostatic force between the residual toner and drum is reduced substantially, the residual toner may be more readily removed from the drum by a magnetic brush or the like.
A drawback exists in this prior art expedient in that the corona discharge is performed in the absence of light. The conductivity of the photoconductive layer in the dark is high, and it is impossible to dissipate the electrostatic charge in the lowermost portion of the layer. Even the subsequent exposure to light does not produce complete discharge, and the problem of double printing is not effectively solved.